1. Technical Field
The present invention relates to an antenna structure capable of performing radio communication in a plurality of different frequency bands and to a communication apparatus including the antenna structure.
2. Background Art
FIG. 11a schematically shows an example of an antenna structure capable of performing radio communication in a plurality of different frequency bands. An antenna structure 1 includes a feeding radiation electrode 2 and a non-feeding radiation electrode 3. The feeding radiation electrode 2 is a λ/4 radiation electrode, and is formed by, for example, a conductor plate. A bent slit 4 including a U-shaped portion is formed in the feeding radiation electrode 2 by cutting the feeding radiation electrode 2 from an electrode edge. One side Q of the two sides of the slit at the edge of the feeding radiation electrode that are separated by the slit 4 serves as a feeding end, and the other side K serves as an open end. An electrode edge connected to the feeding end Q serves as a short-circuited portion Gq for grounding. Due to the formation of the slit 4, the feeding radiation electrode 2 has a folded shape and includes a U-turn portion T in the middle of the path from the feeding end Q toward the open end K.
The non-feeding radiation electrode 3 is also formed by a conductor plate. A bent slit 5 including a U-shaped portion is formed in the non-feeding radiation electrode 3 by cutting the non-feeding radiation electrode 3 from an electrode edge. One side Gm of the two sides at the edge of the non-feeding radiation electrode that are separated by the slit 5 serves as a short-circuited portion for grounding, and the other side 6 of the sides at the edge of the non-feeding radiation electrode serves as an open end. The non-feeding radiation electrode 3 is disposed adjacent to the feeding radiation electrode 2 with a gap therebetween such that the short-circuited portion Gm is adjacent to the short-circuited portion Gq of the feeding radiation electrode 2 with a gap therebetween.
For example, as shown by the return loss characteristics in FIG. 11b, a fundamental resonant frequency F1 0f a resonance that mainly operates due to the feeding radiation electrode 2 is in the vicinity of a fundamental resonant frequency f1 of a resonance that mainly operates due to the feeding radiation electrode 2 and the non-feeding radiation electrode 3 that is electromagnetically coupled to the feeding radiation electrode 2, and the frequencies F1 and f1 produce a complex or dual resonance. In addition, a higher-order resonant frequency F2 of the resonance that mainly operates due to the feeding radiation electrode 2 is in the vicinity of a higher-order resonant frequency f2 of the resonance that mainly operates due to the feeding radiation electrode 2 and the non-feeding radiation electrode 3 that is electromagnetically coupled to the feeding radiation electrode 2, and the frequencies F2 and f2 produce a complex or dual resonance.
The antenna structure 1 shown in FIG. 1 a is capable of performing radio communication in four resonant frequency bands, that is, a fundamental resonant frequency band based on the fundamental resonant frequency F1 and a higher-order resonant frequency band based on the higher-order resonant frequency F2 of the resonance that mainly operates due to the feeding radiation electrode 2 and a fundamental resonant frequency band based on the fundamental resonant frequency f1 and a higher-order resonant frequency based on the higher-order resonant frequency f2 of the resonance that mainly operates due to the feeding radiation electrode 2 and the non-feeding radiation electrode 3 that is electromagnetically coupled to the feeding radiation electrode 2.
The antenna structure 1 is installed on, for example, a circuit substrate of a radio communication apparatus. Thus, the short-circuited portions Gq and Gm of the feeding radiation electrode 2 and the non-feeding radiation electrode 3 are connected to a ground portion of the circuit substrate. In addition, the feeding end Q of the feeding radiation electrode 2 is connected to, for example, a high-frequency circuit 8 for radio communication of the radio communication apparatus.
For example, in the antenna structure 1 shown in FIG. 11a, when a transmission signal is supplied from the high-frequency circuit 8 of the radio communication apparatus to the feeding end Q of the feeding radiation electrode 2, the signal supply causes the feeding radiation electrode 2 to resonate. At the same time, the signal is also supplied to the non-feeding radiation electrode 3 due to electromagnetic coupling, and the non-feeding radiation electrode 3 also resonates. Thus, due to the resonance operation (antenna operation) of the feeding radiation electrode 2 and the non-feeding radiation electrode 3, a signal is radio-transmitted. In addition, when the feeding radiation electrode 2 and the non-feeding radiation electrode 3 resonate (perform an antenna operation) due to an externally arrived signal (radio wave) and receive the signal, the received signal is transmitted from the feeding end Q of the feeding radiation electrode 2 to the high-frequency circuit 8.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 10-93332
In the structure shown in FIG. 11a, the slit 4 is formed in the feeding radiation electrode 2. Electrostatic capacitance is generated in the portion where the slit 4 is formed, and this electrostatic capacitance (C) and an inductance component (L) of the feeding radiation electrode 2 form an LC resonant circuit. The LC resonant circuit is largely involved in a resonant frequency of the feeding radiation electrode 2. Thus, variable control of the resonant frequencies F1 and F2 of the feeding radiation electrode 2 can be achieved by changing the position where the slit 4 is formed, the slit length, and the slit width in order to change a value of the electrostatic capacitance of the portion where the slit 4 is formed and a value of the inductance component of the feeding radiation electrode 2.
However, for example, when the slit length of the slit 4 is increased in order to lower the higher-order resonant frequency F2 of the feeding radiation electrode 2, the fundamental resonant frequency F1 of the feeding radiation electrode 2 is also lowered. Thus, a problem occurs in that it is not possible to lower only the higher-order resonant frequency F2 to a desired frequency. In other words, there is a problem in which it is difficult to individually control the fundamental resonant frequency F1 and the higher-order resonant frequency F2 of the feeding radiation electrode 2.
In addition, when the slit length of the slit 4 is greatly increased in order to greatly lower the higher-order resonant frequency F2 of the feeding radiation electrode 2, the slit 4 may be formed in a spiral shape (coiled shape), for example, as shown in FIG. 12. In this case, the inductance component of the feeding radiation electrode 2 becomes too large, and a signal loss in the feeding radiation electrode 2 becomes large. Thus, radio wave (electric field) radiation is suppressed. In addition, a phenomenon occurs in which electric fields emitted from portions of the feeding radiation electrode 2 cancel each other. If the slit 4 is formed in the spiral shape, the antenna gain of the antenna structure 1 (the feeding radiation electrode 2) is reduced due to the above-mentioned phenomenon.